1. Field of the Invention
The present invention relates to the field of current steering circuits.
2. Prior Art
In various circuits, it may be required for a current to be steered to one of several sub-circuits, depending on one or more logic control bits. By way of example, in a specific product in which the present invention is intended to be used, it is required to be able to selectively turn on any one of multiple output stages of the integrated circuit device, more specifically to turn on one out of three different output stages, by selectively providing a tail current to the output stage intended to be activated. In such circuits it is desired to provide the selective current steering through the use of minimal integrated circuit die area to minimize the cost associated therewith. Further, because of the general desire to operate devices on lower power supply voltages, it is also desired to provide a current steering circuit in which minimal power supply headroom is required. Finally, current steering capabilities without compounding the effects of beta dependent base currents in the steering circuits is desired. The present invention provides a method and apparatus to achieve the goal of minimal supply headroom and minimal die area requirements with minimal beta dependent errors.
Various techniques are well known for steering a single current source to any of multiple outputs. By way of example, a single current source may be connected to the common emitter connection of a first differential pair of transistors so that the differential voltage applied to the bases of the differential pair may be used to controllably direct the current through either transistor of the first transistor pair. The collectors of the transistors of the first transistor pair may be connected to provide the tail current to second and third transistor pairs, a second differential control voltage connected to the bases of the transistors of the second and third transistor pairs determining which transistor in each of the second and third transistor pairs is turned on. In this manner, the first control voltage determines which of the transistors of the first transistor pair is turned on to provide tail current to the second or to the third transistor pair. The second control voltage determines which transistor of each of the second and third transistor pairs is turned on to provide the output current. In this way, the single current source may be controllably steered to the collector circuit of any one of the four transistors in the second and third transistor pairs, as desired. In this circuit, however, the steering circuit increases the headroom already required by the current source being steered by a minimum of 2V.sub.CESAT, where V.sub.CETSAT is the minimum collector-emitter voltage required by a transistor to stay out of the saturation region of operation. Further, assuming the transistors are bipolar junction transistors, the beta dependent base current of whichever transistor of the first transistor pair is turned on adds to (or subtracts from, depending on the conductivity type of the transistor) the current from the current source, so that the collector current of that transistor will be different from that of the current source by the amount of the respective base current. Also, whichever transistor of the second and third transistor pairs is on and conducting collector current of the transistor of the first transistor pair will similarly inject a beta dependent error. Thus, particularly because of the temperature dependent variation in beta of a typical transistor, it is desirable to minimize the number of beta dependent errors which accumulate in the steered current output. Of course, while MOS transistors are not subject to such errors, it is still desirable to minimize the number of MOS transistors a current source must flow through to its ultimate steered current, from headroom and area considerations, particularly when the steered current is large.